FIG. 6 is a sectional view showing a conventional plastic pipe joint and conventional plastic pipes connected thereto. More specifically, one end portion of the plastic pipe joint 100 is connected to the plastic pipe 2A by electric fusion welding, and the other end portion is connected to the plastic pipe 2B similarly by electric fusion welding, so that the plastic pipes 2A and 2B are joined together.
The plastic pipe joint 100 comprises a pipe joint body 101, an electric heating wire 12, and two connector pins 13. The pipe joint body 101 is cylindrical and is made of plastic, and it has an annular inside rim 14, which protrudes from the inner wall at the middle of the joint body 101. The electric heating wire 12 is coiled and buried in the pipe joint body 101. Base end portions of the two connector pins 13 are embedded in the pipe joint body 101 at respective ends thereof, and are connected to respective ends of the electric heating wire 12, Upper end portions of the connector pins 13 protrude from the cylindrical outer wall of the pipe joint body 101. Cylindrical protrusions 16 are formed on the outer wall of the pipe joint body 101 to surround the connector pins 13.
The plastic pipes 2A and 2B and the plastic joint 100 are connected in the following manner. The plastic pipes 2A and 2B are inserted into the plastic pipe joint 100 from both ends until they reach the annular inside rim 14. Then connectors (not shown) are engaged with the connector pins 13 provided at both ends of the plastic pipe joint 100, respectively. Current is applied through the connectors to the electric heating wire 12 in the plastic pipe joint 100 so that heat is generated by the heating wire 12. The heat melts the cylindrical inner wall of the plastic pipe joint 100 and the cylindrical outer walls of the inserted parts of the plastic pipes 2A and 2B, so that the pipes 2A and 2B are welded to the joint 100. Thereafter, the connectors are removed from the plastic pipe joint 100.
In the conventional pipe connecting operation of FIG. 6, the application of current is controlled in the following manner. First, the ambient temperature and the initial temperature of the plastic pipe joint 100 are measured. Then, a period of time for application of current to the heating wire 12 in the plastic pipe joint 100 (hereinafter referred to as "a current application time period") is determined according to the measured temperatures. Current is then applied to the heating wire for the current application time.
One example of the relationship between the current application time period and the initial temperature is shown in FIG. 7. As is apparent from FIG. 7, when the initial temperature ranges from -10.degree. C. to 0.degree. C., the current application time period is 120 sec.; when the initial temperature is in ranges from 0.degree. C. to 10.degree. C., the current application time period is 110 sec.; when the initial temperature ranges from 10.degree. C. to 20.degree. C., the current application time period is 100 sec.; when the initial temperature ranges from 20.degree. C. to 30.degree. C., the current application time period is 90 sec.; and when the initial temperature ranges from 30.degree. C. to 40.degree. C., the current application time period is 80 sec.
The above-described current application control suffers from the disadvantages that it is rather difficult to determine a suitable current application time period for two reasons. First, the current application time period should be determined not only from the initial temperature of the plastic pipe joint but also from the size of the plastic pipe joint and the type of plastic material. Second, the variation in electrical resistance of the heating wire or in the current or voltage applied to the heating wire will offset the temperature and require changes in the current time period.
U.S. Pat. No. 4,684,417 discloses another conventional plastic pipe joint in which a recess is provided in a sleeve of the plastic pipe joint. An electric current is applied to the sleeve, and upon reaching welding temperature, the material of the sleeve expands into the recess. The change of pressure due to expansion of the material or the change of temperature of the material is detected so that the current application time period maybe controlled. However, the shape of expansion of the material is not definite and the detection of temperature is apt to be subjected to the influence of an ambient temperature. Accordingly, the detection is not stable.